Framerate instable when running the program in the background (using syphon to observe)

Hi
I have made a short programm creating waves in one dimension. I am rendering the image in an FBO and then sending it to an other program using syphon. The image it self is never rendered on the screen, just an UI.
However, when I have the window present in front, the image rendered is fine, but when I open another program in front the image becomes wild and as if the framerate goes wild. Do anyone have an idea how I could prevent this from happening.
The code can be found online https://github.com/vertigo-dk/VardeWaves
Thanks in advance
Best
Jonas